Door assemblies having outer panels or skins formed of a fiber reinforced plastic are well known. Such door assemblies typically include a wood frame and an insulative core sandwiched between the outer skins. Further, the skins are typically molded with the outer surface having a wood grain appearance such that the skins can be stained to simulate the appearance of a solid wood door. A number of patents disclose door assembles using reinforced polymer door skins, including U.S. Pat. Nos. 3,950,894; 4,550,540; 4,864,789; 4,720,951; 4,850,168; 4,860,512; 4,901,493; 4,922,674; and 5,142,835.
A number of companies, including the assignee of the present invention, offer commercially a fiber reinforced thermoset plastic door in which the skins are formed by compression molding a sheet molding compound (SMC). SMC typically includes a molding resin of unsaturated polyester polymer blended with a vinyl monomer such as styrene. The SMC includes on the order of 20 to 25 percent by weight glass fiber reinforcement and 10 to 40 percent by weight of inert filler material, typically calcium carbonate. The molded door skins have a relatively large surface area, approximately 18 square feet but are relatively thin on the order of 0.070 inch to 0.120 inch.
The SMC is formed by blending resin and filler to form a resin paste. The resin paste is deposited on a moving plastic carrier film passing directly beneath. Simultaneously, glass fiber rovings are fed into a rotary chopper above the resin coated carrier film. The fibers are cut to length, e.g., 1 inch, and are randomly deposited on the underlying layer of resin. Downstream from the chopping operation, a second carrier film is coated with resin paste and is laid, resin side down, on top of the chopped fibers. This stage of the process creates a resin paste and glass fiber "sandwich" which is then sent to a series of compaction rollers where the glass fibers are wetted out with the resin paste, and excess trapped air is squeezed out of the sheet. At the end of the compaction rollers, the SMC sheet is taken up on a storage roll or bi-folded into a bin. In this stored condition, the roll or folded sheet is wrapped with a barrier film to avoid styrene evaporation.
The SMC resin paste has a viscosity of about 20,000 to 40,000 centipoise when it is deposited on the carrier film. Before the SMC can be used for molding, however, it must age or mature. This maturation time is required to allow the relatively low viscosity resin to chemically thicken. Typically, SMC requires approximately 3 to 5 days to reach the desired molding viscosity. During this time, the viscosity of the material increases such that the material may be cut to the desired length and physically handled by the mold operators. At this point, the SMC sheet reaches about 20 to 30 million centipoise, a consistency similar to leather. SMC continues to thicken after the molding viscosity is reached and therefore has a limited shelf life. Usually SMC must be used within 10 to 14 days from the date of manufacture. If the SMC is not used within this time, its viscosity increases to a point where it can no longer be molded. At this point, the SMC is unusable and becomes waste that must be disposed of.
When the SMC is ready for compression molding, the material is spread onto a cutting table and cut into pieces of predetermined size and shape. The cutting operation is usually done manually with a template and a mat knife by the press operator. The cut pieces are stacked and assembled into a charge pattern that has been determined to be the optimum shape and volume to fill the mold cavity. It is important to assemble the charge pattern as accurately and consistently as possible to avoid process variations.
Generally, the mold is a matched set of forged steel upper and lower die halves that have surfaces that have been formed to create the desired configuration in the molded skins. Typically, this configuration is a flush or paneled door with a wood grain on the outer surface. The steel dies are plated or surface treated to reduce wear, typically with a chrome flash on the surface to aid in release of the molded skins from the die. The mold is heated typically in the range of 300.degree.F. to 350.degree. F.
After charge placement in the mold, the mold is closed by lowering the upper die half, which is mounted to the ram of a large press, onto the lower die half. As the dies come together, the SMC material is compressed therebetween. Pressures for SMC molding of door skins typically exceed 1,000 psi and can go as high as 2,000 psi.
Under heat and pressure, the SMC is transformed from its leather-like quality to a flowable compound. The SMC flows to fill out the mold cavity. Typically, a vacuum is extracted from the mold cavity to aid in flow. During the molding operation, the heat and pressure activate a catalyst in the SMC that causes curing of the thermoset material. The cure time of the SMC and mold varies from 30 to 150 seconds depending on part thickness and material formulation.
After curing, the mold is opened and the door skin is ejected from the lower mold surface with the use of integral ejector pins. The hot skins are handled carefully and usually placed on support racks to cool to ambient temperature. Any edge flash from the part is removed at this time. After the door skin has cooled, it has exceedingly good tensile and flexural strength. The skins are then stacked for shipment.
Compression molding of SMC has a number of disadvantages. One of the major disadvantages is the cost of the dies. Because the pressures involved in the molding operation of door skins typically are in the range of 1,000 to 2,000 psi, the dies must be formed of forged tool steel to withstand these pressures. The weight of the dies for molding of door skins is on the order of 40,000 pounds. The tooling costs are also quite significant. The high pressures involved in SMC compression molding of door skins also require large presses, typically on the order of 2,000 tons. The required presses and forged steel dies thus involve significant size, weight, and cost.
Another disadvantage of SMC, as mentioned above, is that the SMC after being blended and processed with the chopped glass fibers must be set aside to allow for curing of the compound to a viscosity at which it can be handled by the mold operator. This typically requires storage of the material from 5 to 10 days before use. However, if the material is not used, the viscosity continues to increase and the SMC can "overcure". Typically, the SMC must be used within 10 days after it first reaches its desired viscosity. When the material overcures, viscosity rises to a level at which the material may no longer be suitably molded. The SMC at this point is worthless and must be discarded.
A further disadvantage of SMC is that the composition must contain a relatively large amount of a release agent typically zinc stearate to aid in the release of the skins from the dies. Zinc stearate is typically added to the SMC in the range of 5 to 6 percent by weight. However, the zinc stearate as it is intended to do accumulates on the surface of the door skin. The presence of zinc stearate on the outer surface of the door skin detracts from the ability to stain the door. Further, the inner surface of the door is adhesively secured to the door frame. The presence of zinc stearate on the inner surface interferes with this adhesive bond.
Finally, because of the viscosity of the SMC in the mold, it is difficult or impossible to mold the part with openings, e.g., for windows, because the material will not flow around the opening. Typically, when door skins with openings are compression molded, knit lines form where the material flows back together. These knit lines form an area of weakness along which the door may fail. Accordingly, when it is desired to have a door having openings for glass, the cured skins must be cut to form the opening resulting in material waste and additional disposal problems.